Automotive air conditioning systems incorporate a refrigerant expansion valve in the refrigerant line that runs from the condenser to the evaporator, in order to render the pressurized refrigerant suitable for use in the evaporator. Basically, the refrigerant is run through a reduced diameter orifice, causing it to rapidly contract and then expand on the other side, into a low pressure, cold mist. More expensive systems use a selectively expandable and contractible orifice, but a fixed orifice tube, generally brass, is still commonly used, because of its inexpensive and reliable operation. The orifice tube is typically centered within a support plug, which is then crimped inside the refrigerant line to center the tube within the line. The refrigerant leaving and expanding from the downstream end of the tube produces expansion noise, especially evident as a hissing noise at system shut down. Another consideration with expansion valves is the necessity to filter out particulates carried by the refrigerant at the upstream end of the tube.
Conventional orifice tube assemblies deal with upstream particulate filtering and downstream noise attenuation through the use of two different purpose screens, one surrounding each opposed end of the tube. At the upstream end, a relatively large mesh, long and large diameter particulate screen, typically nylon screen, provides enough surface area to catch particulates without totally plugging over a suitable operation interval. At the downstream end, a relatively small mesh, shorter and smaller diameter screen muffles expansion noise, by acting as a baffle to reduce turbulence. Such an assembly, being asymmetrical, is not intended to work in each direction, or to be installed in either direction within the refrigerant line. However, it is possible to accidentally install it backward. If this occurs, the orifice tube per se can perform its task in either direction. Furthermore, the noise attenuation screen, if misplaced upstream, can provide particulate exclusion, albeit for a shorter interval before it would need to be changed, given its smaller total surface area. However, the particulate filter, if misplaced downstream, does not, by its nature, provide adequate noise attenuation, given its larger size, and particularly its larger mesh. Backward assembly will, therefore, be made evident by increased noise of operation, but even if the cause of the noise is properly analyzed, it is inconvenient to have to remove and re install the valve.